Perfecting Injection

Making Maximum Power With Performance Fuel Injectors

Rod Short Feb 3, 2003 0 Comment(s)

When electronic fuel injection first appeared on performance cars, it represented a major shift in the comfort zone of many hot rodders. Turning screws, replacing jets, and setting float levels were all suddenly a thing of the past. Of course, so were the driveability problems sometimes associated with hard acceleration, cornering and stopping. Computer-controlled fuel injection was eventually embraced by gearheads, primarily because of the almost infinite range of tunability. This, in turn, led to the much better results with naturally aspirated performance and with power adders like nitrous, blowers and turbos.

However, modern performance engines often make only as much power as their injectors allow. Taking stock of these often forgotten parts is one of the ways your performance vehicle can make or break a smile.

WHAT INJECTORS DO
In simplest terms, electronic fuel injectors are electro-magnetic solenoids that deliver the proper amount of fuel to your vehicle's engine. These are regulated by the vehicle's on board Electronic Control Module (ECM), which measures the amount of airflow and calculates the amount of fuel needed to obtain the desired air/fuel ratio, and then actuates the fuel injector in just a matter of milliseconds. The ECM then sends a signal to energize the fuel injector solenoid, which allows the fuel to flow.

Several basic factors control the amount of fuel that the injector delivers. The first is pulse width, which refers to the amount of time the injector is open. Next is flow rate, which is measured in lbs/hr for domestic cars and cc's for import. Third is fuel line pressure behind the injector, while the last involves the cleanliness of the injector tip. While the first three are often recognized as contributors to total fuel flow, the latter is less recognized, but equally important, since a dirty injector can impede delivery of fuel.

How the injectors deliver fuel is also important. Some injectors are designed to deliver a wide angle, conical or diffused spray pattern for optimal atomization, burn efficiency and power. Others are set up to deliver a narrower spray pattern to keep the fuel charge from hitting and condensing on the intake and cylinder walls. What's interesting, is while spray patterns can often be checked on a test stand, those patterns can sometimes be distorted in combustion chambers configured to promote swirl or under boost levels with supercharged or turbocharged applications.

Types of Injectors
While injectors are usually chosen by their flow rates, there are also significant differences in terms of resistance and nozzle designs. Different types of injectors should never be mixed or matched as these differences can overheat the ECU driver and drastically affect the ability of your engine to perform.

Injectors are available in a wide variety of flow rates ranging anywhere from 14 lbs/hr up to a whopping 160 lbs/hr, and even more. Knowing how these ratings are achieved, however, is important in making comparisons. Variables involved in determining these rates include duty cycle and fuel pressure. Duty cycle refers to the amount of time the injector stays open. To illustrate this, a performance vehicle at the top end of a drag strip may operate at a 100-percent duty cycle, meaning that the injectors are constantly open and then at 20-percent duty cycle at idle. Most flow ratings, while measured at anywhere from an 80 to 100-percent duty cycle, often use the lower of the two numbers. Fuel pressures, which can vary widely, are often factored in anywhere from 43 to 44 psi when measuring flow rates.

When it comes to understanding injector response, one might compare this to switching a printer on a computer with changing software. Replacement injectors may respond differently to a vehicle's ECU. Most injectors are split up into two categories, either low or high impedance. Low Impedance, which is also known as Peak and Hold, usually have a low resistance of 2 to 3 ohms while High Impedance, or saturated injectors, have a higher resistance of anything from 12 to 16 ohms. Although both types are used in stock production vehicles, it is common to find the cooler operating high impedance injectors in mildly modified vehicles while the quicker reacting low impedance injectors are geared towards all out competition vehicles. While some vehicle's ECU systems are compatible with either type of injector, checking with your supplier before you buy is always a good idea.

Although there are a variety of nozzle designs, the pintle, disc and ball type injectors are by far the best known. Pintle design injectors allow fuel to flow when an upright needle retracts from a tapered seat, while disc injectors feature plates with extremely tiny holes. Ball types, on the other hand, utilize a ball and socket arrangement to control the flow of fuel. While each has its own pro's and con's, all of them have been used successfully in high performance applications. On top of this, there are a variety of O-ring configurations available for sealing along with electrical connections and top or bottom-feed styles.

SIZING INJECTORS
While selecting the correct injector size for performance is something of a science, it can't be emphasized how important it is to get it right. As a rule of thumb, one may use the following simple formula that can be used is as follows:

HP / No. of Injectors X BSFC / Duty Cycle

To illustrate this, if we have a naturally aspirated V-8 engine producing 300 hp at the flywheel, we divide that figure by the number of injectors. Then we multiply that by a Brake Specific Fuel Consumption (BFFC) figure of .50. and divide that by a Duty Cycle of .80. On a calculator, those numbers would look like this:

300hp / 8 injectors = 37.5
37.5 X .50 BFSC = 18.75
18.75 / .80 Duty Cycle = 23.43 (24 lb/hr injector)

What's important to note however, is that the BFSC figure for competition engines can go down to .44 while that same number for forced induction engines can go up to .65. The figure you use for Duty Cycle may change as well. There are a number of very simple calculator programs available to do this math for you. MSD also has a rather sophisticated program (PN2000) for purchase, which takes into account even more variables such as temperature, manifold pressure and rpm.

Since manufacturing methods allow a range of tolerances, some injectors may not flow exactly what the box says they will. That's why performance buyers may ask for a set of flow-matched injectors. Matched injectors, which have been individually tested, hand selected, and matched to within one percent of each other, help insure that each cylinder is getting the same amount of metered fuel.

KEEPING UP WITH INJECTORS
Once a person has selected the right style and size injector, that doesn't mean they can be completely forgotten. Contamination and heat are an injector's worst enemy, and can not only cause an engine to run poorly, but also possibly flunk an emissions inspection.

Small particles of dirt, fuel cell insulation, and even minute pieces of metal have been known to internally clog injector nozzles, resulting in irregular spray patterns. When an engine shuts off, small amounts of fuel adhering to the injector tips can also break down and leave deposits, especially in cases where high temperatures are involved. In cases like these, the cylinder serviced by this injector will run abnormally lean, which can cause the vehicles ECU to overcompensate by causing an overly rich condition, which will drastically increase harmful pollutants. Leaky injectors, on the other hand, can cause the reverse situation.

The only answer to situations like these is to clean the injectors either on or off the vehicle, or replace them all together. Adding a bottle of fuel injector cleaner to a tank a gas periodically is great preventive maintenance, but often does little to clean deposits that have already set in. Other methods may include disabling the fuel pump and then attaching a pressurized can of high strength cleaner to the fuel rail. The car is then started, and runs off the can of cleaner until it's exhausted. While this method usually works well, others opt to remove all their injectors to have them sent in, cleaned and rebuilt. Doing this regularly can often add up to more than the cost of a set of new injectors, however. Investing in a set of performance injectors might be a good alternative, even if they are the stock replacement size.

With better-than-stock materials and construction, that might be one of the least expensive insurance policies you'll ever find for your engine's performance.

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This cutaway shows the Bosch style pintle style nozzle that ACCEL utilizes with each of their injectors. Each ACCEL injector also features an anti-plugging vapor cap and the flow rating for each injector which is etched into the molded top. Precision molded O-rings and fine mesh fuel strainers are also included in each injector size ranging from 14 to 48 lbs/hr.

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While injector manufacturers may use a variety of spray nozzles, some of the more important things in selecting an injector is knowing how they are rated, how much fuel is required for an engine's horsepower, and how closely the fuel rates have been matched.

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Large scale Injector Flow benches likes these are used to check injector operation and flow rates with different fuel pressures, duty cycles and even ignition systems. The injectors from this bench will be logged, sorted and then hand selected so the flow rates are within 1 percent of each other.

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Selecting an injector with the right spray pattern depends on where it's mounted. Patterns that are too wide may cause fuel to drop out of air suspension, condense on the walls of the runner, and run down. Narrower-shaped patterns aimed at the intake valve, sometimes work best with wet intake manifolds that have been converted over to fuel injection.

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While there are a number of ways to clean injectors, removing them from the vehicle and then cleaning them on a jig like this often gives the best results. This also gives the technician an opportunity to inspect other components such as O-rings and electrical connectors for proper operation.

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